In olefin epoxidation an olefin is reacted with oxygen to form an olefin epoxide, using a catalyst comprising a silver component, usually with one or more further elements deposited therewith on a support. The olefin oxide may be reacted with water, an alcohol, carbon dioxide or an amine to form a 1,2-diol, a 1,2-diol ether, 1,2-carbonate or an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers, 1,2-carbonates and alkanolamines may be produced in a multi-step process comprising olefin epoxidation and converting the formed olefin oxide with water, an alcohol, carbon dioxide or an amine.
The performance of the epoxidation process may be assessed on the basis of the selectivity, the catalyst's activity and stability of operation. The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. The catalyst is subject to an ageing-related performance decline during normal operation. The ageing manifests itself by a reduction in the activity of the catalyst. Usually, when a reduction in activity of the catalyst is shown, the reaction temperature is increased in order to compensate for the reduction in activity, however at the expense of selectivity. In the typical operation of a fresh catalyst, the process is operated at a reaction temperature of up to about 250° C. Upon catalyst ageing the reaction temperature may gradually be increased to values substantially above 250° C. until the reaction temperature becomes undesirably high or the selectivity becomes undesirably low, at which point in time the catalyst is deemed to be at the end of its lifetime and would need to be exchanged. It goes without saying that from an economical point of view it is highly desirable to improve the performance of the catalyst and to extend its lifetime as much as possible. Quite modest improvements in the maintenance of selectivity over long periods yield huge dividends in terms of efficiency in the olefin epoxidation process and, if applicable, also in the overall process for the production of a 1,2-diol, a 1,2-diol ether, 1,2-carbonate or an alkanolamine.
Therefore, for decades much research has been devoted to improving the activity, the selectivity and the lifetime of the catalysts, and to find process conditions which enable full exploitation of the catalyst performance. For example, it is well known that low CO2 levels are useful in improving the selectivity of high selectivity catalysts. See, e.g., U.S. Pat. Nos. 7,237,677; 7,193,094; U.S. 2007/0129557; WO 2004/07873; WO 2004/07874; and EP 2,155,708. These patents also disclose that water concentration in the reactor feed should be maintained at a level of at most 0.35 mole percent, preferably less than 0.2 mole percent. Other patents disclose control of the chloride moderator to maintain good activity. See, e.g., U.S. Pat. No. 7,657,331; EP 1,458,698; and US Pub. Pat. App. 2009/0069583. Still further, there are many other patents dealing with the epoxidation process and means to improve the performance of the catalyst in the process. See, e.g., U.S. Pat. Nos. 7,485,597, 7,102,022, 6,717,001, 7,348,444, and US Pub. Pat. App. 2009/0234144.
Notwithstanding the improvements already achieved, there is a desire to further improve the performance of the silver-containing catalysts in the production of an olefin oxide, a 1,2-diol, a 1,2-diol ether, a 1,2-carbonate or an alkanolamine.